1. Field of the Invention
The present invention relates to spherical silica-containing compound oxide particles useful as, for example, a filler in epoxy resin sealants for IC devices, an abrasive, a conductive agent, a photo-catalyst, an additive for transparent film, and a light-transmissive additive for antireflection film in liquid crystal display devices. The invention also relates to a method of producing such particles, and the use of such particles as an additive for modifying an index of refraction.
2. Prior Art
Several methods for preparing silica-containing compound oxide particles are known in the art. Silica-containing compound oxide particles are prepared, for example, by a method of heating a chlorosilane and a metal chloride for evaporation and effecting hydrolysis in a flame (Japanese Patent No. 2,503,370 corresponding to U.S. Pat. Nos. 5,672,330 and 5,762,914), a method of atomizing and burning a slurry of silica and metal oxide powders dispersed in a combustible liquid (JP-A 10-297915), a method of adding an aqueous colloid sol of dispersed metal oxide microparticulates to a porous spherical silica gel, causing aggregation and drying (JP-A 6-127932), a method of emulsifying a dispersion of titanium dioxide microparticulates in an aqueous alkali silicate solution in an organic solvent with the aid of a surfactant, to thereby form an emulsion, blowing carbon dioxide gas therein, and gelling dispersion droplets (JP-A 11-322324), and a method of contacting silicon tetrachloride in liquid or gas form with titanium dioxide powder for forming a hydrolyzate on particle surfaces (JP-A 7-247118).
However, the method of subjecting chlorosilane and metal chloride to flame hydrolysis has the drawbacks that the chlorine contained in the chlorosilane and metal chloride is not fully removed, and that since the feed stock is to be evaporated by heating, it is limited to a metal chloride which can be evaporated by heating under atmospheric pressure. Further, the method of atomizing and burning a slurry of silica and metal oxide powders in a combustible liquid has the drawback that since silica and metal oxide have different melting points and the powders have an uneven particle size distribution, the start point of melting differs between them, failing to produce a uniform compound oxide. The method of adding a colloid sol of metal oxide to a porous spherical silica gel and drying into a gel has the drawbacks that it takes a time for the colloid sol to fully penetrate into the porous material and for the penetrated colloid sol to dry up, and that the sol undergoes volume shrinkage upon drying, often leaving voids in pores. The method of dispersing titanium dioxide microparticulates in an aqueous alkali silicate solution and emulsifying it in an organic solvent, followed by gelling, enables surfaces of titanium dioxide to be coated with a silica film, but has the drawbacks that it is difficult to uniformly incorporate titanium dioxide within silica and that the alkali metal is left behind. The method of contacting titanium dioxide with silicon tetrachloride for hydrolysis has the drawback that chlorine is left behind.
Meanwhile, several methods of modifying the index of refraction of powder are known in the prior art. Known methods include (1) a method of mixing two or more oxides having different indices of refraction, melting the mixture, cooling the melt for solidification, and grinding the mass into a powder, (2) a method of forming a compound oxide mass by the sol-gel process and grinding the mass, (3) co-hydrolysis of different metal alkoxides in liquid phase, (4) vapor phase hydrolysis of metal alkoxide, and (5) hydrolysis of a vapor mixture of metal chlorides in a flame.
However, the powdering method (1) involving melting and grinding has the problems that the composition tends to undergo segregation upon cooling of the melt, the particle size cannot be reduced below a certain limit, and impurities are entrained during the grinding. The method (2) of grinding the compound oxide mass resulting from the sol-gel process also has the problems that the particle size cannot be reduced below a certain limit, and impurities are entrained during the grinding. The method (3) of metal alkoxide co-hydrolysis in liquid phase has the drawbacks that dropwise addition and reaction is time consuming, the necessity of post-heating at elevated temperature increases the cost, and a limit is imposed on the composite composition in order to prevent crystal grains from growing in the heating step. In the method (4) of evaporating metal alkoxides and hydrolyzing them in a vapor phase, those metal alkoxides having a high boiling point do not develop a high vapor pressure, a limit is imposed on the composite composition, and the resulting particles are ultrafine, with few particles of more than 50 nm in size being available. In the method (5) of hydrolyzing a vapor mixture of metal chlorides in a flame, the metal chloride used are limited to those having a low melting point and a limit is imposed on the composite composition. Chlorine left in the product is also a problem.